Electronic devices such as computers, personal digital assistants, portable telephone devices and the like all need input means for enabling a human user to input commands and other information. The most commonly used input means comprise key pads and touch pads. Both of these involve the concept of producing an electric signal as a response to a pressing movement made by the user; the difference between the two is mainly in that a key pad only offers a relatively small number of discrete pressing points or keys to be pressed, while a touch pad is capable of differentiating between a large number of pressing locations and can even provide truly continuous detection of movement of the depression across the pressable surface. The borderline between the two is not always very obviously discernible.
FIG. 1 is an exploded cross-sectional view of a representative prior art key pad structure. A supporting structural part of the key pad is a printed circuit board 101. On its top surface it has a layer 102 of conductive patterns that at certain locations constitute patches 102a of intertwined fingers. The conductive patterns are covered with a perforated insulating layer 103, the holes of which coincide with the patches 102a. On top of the conductive layer there is the so-called domesheet 104, the domes of which are conductive on their concave side. Each dome is covered with an elastic key hat 105, and the whole arrangement is enclosed within an outer cover part 106 so that the key hats 105 protrude through respective holes in the outer cover part 106. Pressing a key hat 105 will cause a central part of the dome under it to bulge downwards, so that it forms an electrically conductive connection at one of the patches 102a. A piece of reading electronics (not shown) is coupled to the conductive patterns 102 and adapted to detect, at which patch 102a did the connection occur. Said reading electronics translate this location information into an unambiguous electronic input signal.
Touch pad input means come in a large variety of types, even including technical solutions that are relatively far from each other. FIG. 2 illustrates schematically a layered structure that can be used, with few modifications, for realizing a resistively coupling or a capacitively coupling touch pad. On the top surface of a printed circuit board 201 there is an essentially uniform resistive layer 202, the opposing edges of which are equipped with conductive electrodes 203 and 204. The term “resistive” is used to describe a material that is neither an insulator nor a conductor, but has a specific electric resistance that can be utilized to detect, how far did an electrical current have to propagate within said material. On top of the resistive layer 202 there comes an isolation web 205 and a second resistive layer 206, which also has conductive electrodes 207 and 208 running along its opposite sides. The electrodes 207 and 208 of the second resistive layer 206 are located at different sides of the generally rectangular form of the resistive layers 202 and 206. The whole arrangement is covered with a protective outer layer 209.
In the touch pad of FIG. 2 the isolation web 205, the second resistive layer 206 and the outer layer 209 are all elastically deformable to some extent. When the user presses some point of the outer layer 209 with the point of a stylus or other pressing means, an essentially point-like elastic deformation occurs that causes that point of the second resistive layer 206 to approach the first-mentioned resistive layer 202. In a galvanically coupling touch pad the deformation is large enough to cause the resistive layers to actually touch each other, while in a capacitively coupling touch pad it suffices to locally reduce the distance between the resistive layers so that the local change in capacitance between the resistive layers is large enough to be detected. In any case a piece of detection electronics (not shown) is coupled to the electrodes 203, 204, 207 and 208 and adapted to measure the resistive or capacitive characteristics of the created connection. Correlating the two simultaneous measurements obtained from the resistive layers 202 and 206 it is possible to deduce the location at which a depression occurred.
Using continuous electrodes along the whole length of sides of the resistive layers is not the only possible alternative for coupling the resistive layers to reading electronics. Some other solutions employ discrete conductive wires connected to specific points along the sides of the resistive layers, or even to certain intermediate points within the resistive layers.
More seldom encountered touch pad technologies involve e.g. detecting a change in acoustic vibrations caused by the pressing means, or using a light-emitting stylus and observing the way in which light coming from the stylus tip propagates in a light guide that forms a part of the layered structure of the display. It is highly probable that new technological ideas continue to surface regarding the fundamental way in which an active layer translates a touch or a depression into an electrical signal representing its location of occurrence.
The drawbacks of prior art key pads and touch pads are mainly related to the relatively large number of separate components and/or method steps required in their manufacture. As an example we may consider the touch pad structure of FIG. 2. The printed circuit board 201, the isolation web 205, the second resistive layer 206 and the protective outer layer 209 are all separate components that must be manufactured, brought into an assembling machine, aligned properly, and attached to each other. Electrical connections must be established from the electrodes 203, 204, 207 and 208 to the reading electronics. For lowering the manufacturing costs in mass production a simpler structure would be desirable.